Dissolution of the Tetrarchy

The Dissolution of the Tetrarchy was a period of civil war in the Roman Empire from 306 to 324 AD during which the Tetrarchy system, established by the abdicated emperor Diocletian, violently collapsed as rival dynasties attempted to restore the dynastic succession system. Constantius I's son Constantine the Great defeated Maximian's son Maxentius at the Battle of Milvian Bridge in 312, and he then defeated Licinius at the Hellespont in 324 AD, uniting the empire under his dynasty's rule.

Background
Following the death of the tetrarch Constantius I, his son Constantine I took his place as emperor, a succession which was not supposed to occur under Diocletian's Tetrarchy system. Maxentius, the son of the abdicated Eastern Roman emperor Maximian, felt bypassed for a place in the imperial college at his father's abdication, as Diocletian had sought to prevent relatives from succeeding their fathers as emperors, and Constantius broke the rule. After Constantine's army acclaimed him as ruler, Maxentius urged the Praetorian Guard of Rome to proclaim him emperor, and he took over Rome and Italy, turning the Tetrarchy into the "rule of five"; Maxentius then brought his father Maximian out of retirement to assist him, making it "rule of six".

Civil war with Maxentius
The other tetrarchs Galerius, Severus, and Constantine all considered Maxentius a usurper, and knew that there was no other option but civil war. Galerius directed Severus to take his army into Italy and lay siege to Rome, but many of Severus' soldiers defected to the opposing side and he was defeated in battle, personally captured, and forced to commit suicide. Galerius then decided to attack Maxentius from the east, but he too failed to complete the mission, as he was unable to besiege Rome or maintain the loyalty of his troops; he fled Rome and barely managed to escape alive. During all this time, Constantine was campaigning against barbarian threats in the north and letting the other leaders of the empire fight it out among themselves. For six years, Constantine observed as famine and food shortages resulted in riots in Rome, soldiers rampaged through the city, the law was twisted with unjust imprisonments and executions, and reports of Maxentius' own profligate activities became known. In 312 AD, Constantine decided that the time was right to invade Italy, seeking to both overthrow the tyrant in the capital and to gain possession of Rome, all of Italy, and North Africa. Constantine accompanied his army over the Alps and destroyed all resistance en route. He outflanked and destroyed Maxentius' cavalry forces at Augusta Taurinorum (Turin) and achieved the loyalty of several Roman cities, and he also captured Verona after defeating a sortie attempt. He moved his army south within striking distance of Rome in preparation for an ultimate battle with the usurper, after which he would take control of the entire Italian peninsula. It was then that he had a vision, in which he was instructed to paint the symbol of the Christian God on the shields of his soldiers in order to achieve victory; they painted ☧, which symbolized the name of Jesus "Christ".

Battle of Milvian Bridge
Constantine had several advantages which his opponents lacked: Maximian had committed suicide under duress from Constantine two years earlier, depriving Maxentius of a brilliant commander. Maxentius had all the bridges that crossed the Tiber River destroyed, making engagement more difficult for an attacking army, but he chose to fight the upcoming battle on an open field rather than steel the city against a coming siege. They built a pontoon bridge, the Milvian Bridge, and engaged Constantine's army in battle. Cornered against the river, they were routed, and the bridge collapsed under the weight of the fleeing soldiers, and Maxentius was among those who drowned. Constantine entered Rome the next day as its new ruler and the emperor of the Western Empire.

Constantine versus Licinius
The death of Severus years earlier led to the general Licinius becoming the new emperor of the West. Galerius died soon after Milvian Bridge, and his successor Maximinus II was defeated in battle by Licinius not long after. The Tetrarchy was destroyed, as Severus, Galerius, Maxentius, and Maximinus Daia were all dead, leaving Constantine in the west and Licinius in the East. They decided to broker a peace and met in Milan to cement their friendship, with Licinius marrying his half-sister Constantia; they also both agreed to end the persecution of Christians. The Edict of Milan, addressed to provincial governors in the East, was neither written in Milan nor a co-authorship (Licinius wrote it), but both men were credited, and it legalized the Christian faith. By 313, Constantine and Licinius had reached a temporarily amicable decision to co-rule the empire. However, Constantine was a man never satisfied, and, over the next deecade, occasional military flare-ups between Constantine and Licinius' forces led to frayed relations. In 324 AD, Licinius decided to renew the persecution of Christians in the East, leading to Constantine's declaration of war against Licinius to rescue his co-religionists. He defeated Licinius at the Hellespont, sent him to retirement in Thessalonica, and eventually ordered his death, giving him complete control of Rome. In 326 AD, he celebrated his 20th anniversary as emperor, and he decided against sacrificing to the Roman god Jupiter, incensing pagan members of the Roman Senate. He began to build a "New Rome" in the east, far away from pagan influences, and this explicitly Christian city became known as Constantinople. Rome was not just unified again, but was now ruled by a Christian emperor.